Yellow fever virus (YFV) is a mosquito-borne emerging/re-emerging hemorrhagic fever virus that causes 20- 60% mortality and is endemic in >40 countries. The current live attenuated YFV vaccine was developed in 1936 and has proven to be effective at saving millions of lives from this devastating disease. Nevertheless, this is a live-attenuated vaccine that is contraindicated in healthy people who have egg allergies as well as vulnerable populations including young infants, pregnant or breastfeeding women, and the elderly. During recent outbreaks, these at-risk groups have had no alternatives to live yellow fever vaccination and our goal is to produce a vaccine that is safe for both healthy and vulnerable populations. According to the CDC, live YFV vaccines cause 47 serious adverse events (SAE) per million vaccinations (SAE defined as resulting in hospitalization, long-term disability, or death). Vaccine-associated neurological disease occurs at a rate of up to 1 case per 10,000 vaccinations. YFV vaccination of infants <9 months of age has been contraindicated since the 1960?s due to excessively high rates of vaccine-associated encephalitis in this age group. More recently, live YFV vaccination has been contraindicated in breastfeeding mothers due to documented cases of virus transmission via breastmilk to infants who later developed YFV-associated neurological disease including seizures. In patients >60 years of age, YFV vaccination causes severe viscerotropic disease at an incidence rate of approximately 1:50,000 with a mortality rate of >50%. The overall mortality rate following YFV vaccination (all ages) is estimated at 1 to 2 deaths per million doses. Despite these clear gaps in vaccination coverage, there is currently no commercial vaccine available for these vulnerable populations. To address this critical unmet need, we have discovered a safe and immunogenic peroxide-inactivated yellow fever vaccine, HydroVax-YFV. Importantly, this advanced vaccine is safe and provides complete protection against lethal viscerotropic yellow fever in a robust non-human primate model. Here, we propose a double-blind, randomized, placebo-controlled Phase I dose escalation trial to evaluate the preliminary safety and immunogenicity of HydroVax-YFV. Our goal is to eventually expand vaccine coverage to a broader range of patients and the successful completion of this study will represent a key milestone in the advancement of a clinically relevant vaccine against yellow fever and provide a much-needed approach to protect the most susceptible members of society including infants, elderly, and those with potentially compromised immune functions.